WordPress and Site.pro are completely different website builder systems. And migrating a website from one platform to another is very difficult in most cases. So if you are going to switch from Site.pro to WordPress, please be prepared to rebuild your site from scratch.
If you are going to switch site platforms on the same account and domain, you may want to make sure the old website is fully removed before setting up a new one. Something like this:
That said, from what I could find on the web regarding that error message, as far as I can tell it’s “something” WordPress related. It would help a lot if we could check your site for problems. Could you please share the URL to your site so we can check it?